(55637) 2002 UX25 is a Spitzer dwarf-planet candidate that orbits the Sun in the Kuiper belt beyond Neptune. Its orbit takes roughly 280 years, and it has one known moon. This moon makes it much easier to calculate mass, and when the size is known, also a density. The density has been a surprise to astronomers, about 0.82 g/cm3.[13]

The discovery of a satellite of 2002 UX25 was reported in IAUC 8812 on 22 February 2007.[4] The satellite was detected using the Hubble Space Telescope in August 2005.[4] The satellite was found at 0.16 arcsec from the primary with an apparent magnitude difference of 2.5.[15] It orbits the primary in 8.3 days, with a semimajor axis of 4770±40 km, yielding a system mass of 1.25×1020 kg.[5] Assuming the same albedo as the primary, the magnitude suggests the satellite has a diameter of 190 km; assuming a 5% albedo (typical of other similar sized, cold, classical KBOs) suggests a diameter of 260 km.[5]

2002 UX25 is one of the largest known solid objects in the Solar System that is less dense than water. Why this should be is not well understood: objects of its size in the Kuiper belt are typically rocky and dense, and to have a similar composition to others of its kind, it would have to be exceptionally porous, which is unlikely given the compactability of water ice.[5] Its density of 0.82 g/cm3 shocked astronomers.[13]